1. Field of the Invention
This invention relates to a semiconductor laser driving apparatus, and more particularly to a semiconductor laser driving apparatus useful in an optical disk recording and reproducing system wherein information is recorded on and reproduced from a optical disk such as a magnetooptical disk.
2. Description of the Prior Art
In an optical disk recording and reproducing system, information is recorded in and reproduced from an optical disk such as a rewritable type magnetooptical disk. Hereinafter, description will be made, taking a rewritable type magnetooptical disk as a typical example of an optical disk. However, the invention can be also applicable to a semiconductor laser driving apparatus for an optical disk of another type such as a write-once type one or phase-transition type one.
When information is to be recorded on or erased from a magnetooptical disk comprising a magnetic thin film, a high power laser light beam is irradiated onto the disk to elevate locally the temperature of the magnetic thin film which has been perpendicularly magnetized, thereby causing the magnetization of the magnetic thin film to invert in the direction of an external magnetic field. In contrast, when information is to be reproduced from the magnetooptical disk, a low power laser light beam is irradiated to the disk to detect the variation in a reflected light beam polarization which corresponds to the state of the magnetization of the magnetic film. Hence, an optical disk recording and reproducing system is provided with a semiconductor laser driving apparatus which supplies a driving current to a semiconductor laser device the level of which is controlled in accordance with the operating modes of the system (i.e., the recording and erasing mode or the reproducing mode).
FIG. 7 shows a prior art semiconductor laser driving apparatus. In the reproducing mode, a switch circuit 21 is closed by the reproduction ON signal. Then, the output of an operational amplifier 22 to which a power source V.sub.ref is connected via the non-inverting input, is supplied to a transistor Tr to turn it ON, thereby a reproduction driving current I.sub.R is supplied to a semiconductor laser device 23 to emit a laser light beam. The laser light beam is monitored by an optical detecting element 24 such as a photodiode so that a signal, the level of which corresponds to the power of the laser light beam, is supplied to the inverting input of the operational amplifier 22. This negative feedback enables the transistor Tr to control the reproduction driving current I.sub.R so that the power of the laser light beam emitted from the laser device 23 is maintained to a predetermined level.
Because the optical output level of the semiconductor laser device 23 is influenced by the change in temperature, it is not sufficient for maintaining the output power of a laser light beam at a fixed value to control the reproduction driving current I.sub.R at a constant level. This will be described in more detail, with reference to FIG. 8. When the temperature changes, the I-P (driving current-optical output power) characteristic of the semiconductor laser device 23 greatly changes, for example, from curve A to curve B. In accordance with curve A, a fixed optical output level P.sub.R can been obtained by supplying a reproduction driving current I.sub.R1 to the laser device 23. When the temperature rises, the I-P characteristic of the laser device 23 changes as shown by the curve B, resulting in that a greater reproduction driving current I.sub.R2 is necessary for obtaining the optical output level P.sub.R. In order to maintain the optical output level P.sub.R at a constant level, therefore, the reproduction driving current I.sub.R2 must be controlled while monitoring the optical output level P.sub.R.
When the recording and erasing mode is set, the reproduction driving current I.sub.R is fixed to a level which is same as the one at the time immediately before setting the mode, by a sample hold circuit (not shown). A record and erasing signal circuit 25 produces a recording and erasing signal. In accordance with the recording and erasing signal, another switch circuit 26 is closed or opened. When the switch circuit 26 is closed, a record and erase driving current I.sub.W flows through the semiconductor laser device 23 while being superposed on the reproduction driving current I.sub.R. Consequently, when information is recording, the laser light beam emitted from the laser device 23 is modulated in accordance with information to be recorded. When information is erasing, the switch circuit 26 remains to be closed to allow the record and erase driving current I.sub.W to flow through the laser device 23.
The amount of the record and erase driving current I.sub.W changes in accordance with the resistance value of the resistor circuit 28 which is selected by a selection circuit 27. The selection circuit 27 has four switches which can be selectively closed or opened in response to a selection signal supplied from an optical output power selection signal circuit 29. The resistor circuit 28 has four resistors R.sub.1 to R.sub.4 which have a different resistance value and are respectively connected in parallel to the four switches of the selection circuit 27. The selection signal circuit 29 detects the position of an optical disk on which the laser light beam of the laser device 23 is irradiated, i.e., the distance between this position and the center of the disk (hereinafter, such a position is referred to as "an irradiated position"), and produces the selection signal which corresponds to the detected position. The signal circuit 29 produces the selection signal so that the level of the record and erase driving current I.sub.W becomes greater as the irradiated position moves outwards (i.e., towards the outer periphery of the disk).
The reason why the level of the record and erase driving current I.sub.W changes in accordance with the irradiated position is that, when an optical disk rotates at a constant angular velocity, the linear velocity of the irradiated position becomes faster as the irradiated position moves outwards. In other words, in order that the energy of the laser light beam given to the magnetic thin film is kept constant regardless of the irradiated position, it is necessary to increase the optical output power of the laser light beam as the irradiated position moves outwards. The resistors R.sub.1 to R.sub.4 are selected by the selection circuit 27 so as to increase stepwise the record and erase driving current I.sub.W as the irradiated position moves outwards.
A semiconductor laser driving apparatus having such a configuration is described in the Japanese Laid-Open Patent Publication (kokai) No. 62(1987)-257,640.
Thus, in a conventional semiconductor laser driving apparatus, the level of the record and erase driving current I.sub.W which is superposed on the reproduction driving current I.sub.R is always constant as far as the irradiated position remains still.
As mentioned above, the I-P characteristic of the semiconductor laser device 23 changes when the temperature rises. The change of the I-P characteristic due to the temperature change is not a mere shift of the characteristic curve (e.g., from curve A to curve B' as shown in FIG. 8), but is one which includes the change in the slope of the characteristic curve (e.g., from curve A to curve B as shown in FIG. 8). When the I-P characteristic is changed from curve A to curve B, the differential efficiency is reduced from .DELTA.P.sub.X1 /.DELTA.I.sub.W to .DELTA.P.sub.X2 /.DELTA.I.sub.W. If, according to the characteristic curve A, a predetermined optical output level P.sub.X1 has been obtained by superposing the record and erase driving current I.sub.W on the reproduction driving current I.sub.R1, the temperature rise causes the laser device 23 to operate in accordance with curve B, resulting in that the optical output reaches only the output level P.sub.X2, even when the record and erase driving current I.sub.W is superposed on a reproduction driving current I.sub.R2 which maintains the optical output level P.sub.R for the reproduction at a constant value. This is caused by the reduction in the optical output of the laser device which is due to the decrease in the differential efficiency. When the temperature falls, conversely, the optical output level of the laser device 23 may be excessively increased.
In this way, a prior art semiconductor laser driving apparatus cannot cope with the change in the differential efficiency of the I-P characteristic of a semiconductor laser device, and has a drawback that it cannot control the laser device to emit the laser light beam having the optimum power.
As the differential efficiency may differ depending upon individual semiconductor laser devices even at the same temperature, a prior art semiconductor laser driving apparatus has another drawback in that the combinations or resistance values of the resistors R.sub.1 to R.sub.4 in the resistance circuit 28 need to be initially adjusted for each apparatus.
The above-mentioned drawbacks are generally applicable to a semiconductor laser driving apparatus for other optical disk systems including those for a write-once type optical disk.